For mobile telephones that are currently on the market, as examples of communication terminals, the relevant GSM Standard specifies that a standard net bit rate be used for the speech coder which carries out the function of coding of speech signals to form reduced-data narrowband speech signals, which are transmitted via a transmission output stage of the communication terminal. The GSM-specific net bit rate is 12.2 kbit/s, and relates generally to the widely used EFR Codec. Further developments of the EFR Codec have the aim, however, of making it possible to process different net bit rates for the speech coder. In this context, the NBAMR Codec (“Narrowband Adaptive Multirate”) is commonly used which, overall, allows eight different net bit rates for operation of the speech coder, (4.75; 5.15; 5.9; 6.7; 7.4; 7.95; 10.2 and 12.2 kbit/s). In this case, the expectation from the lower net bit rates is that they will actually have advantages for comparatively poor radio transmission paths, since the transmitted signals can be provided with greater redundancy.
The measure of providing a communication terminal with a bandwidth expansion device which evaluates a narrowband speech signal (which has been received by the communication terminal) and synthesizes at least one further frequency band on the basis of the evaluation by means of a suitable algorithm is likewise known from the prior art. Normally, the currently used narrowband speech signal is in the frequency band between 300 Hz and 3.4 kHz. Additional frequency bands can be produced by synthesis both at the low-frequency end and at the high-frequency end of this frequency band, thus resulting in bandwidth expansion. Communication terminals such as this with bandwidth expansion have, however, so far been proposed only in conjunction with speech coders which operate at a single net bit rate.